Hi all,

I have a 1983 240 and lately it's been sounding pretty bad when idling. When I first start the car up, it'll idle fine but after driving around a little, the car sounds like it's about to stall. It'll sound like the RPM's go down then up in waves. When I put the car into drive, instead of moving at a constant speed, it'll move a little faster or slower depending on what point of the "wave" it is (faster when it's idling a little higher, slower when it's idling lower). I don't have a clue what this might be, and the car has been neglected in the past so almost nothing has been changed. I was thinking maybe sparkplugs, but I'm not sure. Does anyone have any ideas?

 

Check all the basics first because you can't get a good scope of you have bad spark plugs, and they sound like they need to be changed anyway.

Spark plugs
Cables
Distributor cap
Rotor


Next check things like the IAC valve (idle air control) valve as although I have not seen one fail out right, they are almost always dirty

A leak may also cause weird idle problems, then the IAC valve may try to compensate resulting in a "wave" like idle (rather than just a bad constant idle)

 

air intake leaks between the MAF (or k-jet air meter), throttle body, and intake manifold, including injector seals and the idle air control valve hoses, can make for really erratic idling. one way to detect these is to use a propane torch like a bernzomatic, and 'flood' the area with unlit propane, if the idle changes, odds are there's a leak nearby.

but, definitely, catch up with basic maintenance items like the ignition system stuff listed above before you go chasing more esoteric stuff.